1. Field of the Invention
The present invention is directed generally to a novel method and apparatus for isolating and, in some cases, anchoring an implantable electrically stimulating probe such as a defibrillator electrode for use with an automatic implantable cardioverter/defibrillator (AICD). More particularly, it relates to the design of the bio-compatible conducting coating or covering or a pouch for containing an electrode which minimizes tissue ingrowth, reduces the current density applied to adjacent tissue, and which, in the case of the pouch, is designed to be fixed to the adjacent tissue in a manner which allows the probe to be inserted and removed using a minimally invasive procedure.
2. Discussion of the Relevant Art
Automatic implantable cardioverter/defibrillator devices have been under development for some time. The term cardioverter is used to mean a device for the correction of either ventricular tachycardia (abnormally rapid heart rate, i.e. 120-180 beats per minute) or ventricular fibrillation (an extremely rapid heart beat disorder) by discharging electrical energy into the heart normally between internally placed electrodes. The electrode arrangement may include a catheter or endocardial electrode which is intravenously positioned within the heart of the patient so that the electrode is within the right ventricle. The other electrode, in the form of a flexible, substantially planar patch, is positioned outside of the heart, either within the thoracic cavity next to the left ventricle, or subcutaneously. The current utilized for the devices is supplied by a battery powered pulse generator implanted under the skin of the patient. Improving the conductance path between the patch electrode and the catheter electrode results in reduced electrical use per defibrillation pulse and this, of course, increases the time that the system can operate without renewal of batteries. Thus, it has also been an advantage to place the defibrillation patch electrode as close to the outer tissue of the left ventricle as possible to shorten the required conduction path.
Placement of the electrode directly on the tissue, however, can result in burns, edema and other tissue injury trauma because of the proximity of the high energy discharge associated with a defibrillation pulse. In addition, tissue adhesion or tissue ingrowth or through growth associated with placement of the electrode directly on the tissue, has resulted in severe removal problems after chronic placement.
As used throughout the specification, the term "dissection plane" is intended to mean any internal boundary which is easily separated even though the adjacent surfaces may be touching. This includes the surfaces of internal organs, for example, which may touch, cooperate and function in harmony but which do not grow together. The term "ingrowth" refers to the cellular penetration of a surface which impedes or hinders its ability to exist as a dissection plane. Specifically, ingrowth refers to cellular penetration of fibrous or connecting tissue through an open or porous surface of a chronically implanted device. The term "adhesion" refers to the surface attachment of fibrils of collagen or the like which can be separated without cutting or incising the tissue.
Previous defibrillation patch electrodes such as those placed on or adjacent to the outer wall of the left ventricle of the heart, have, in some cases, attempted to incorporate an insulating backing material to minimize tissue ingrowth. The insulating layer in the electrode, of course, limits the amount of exposed electrode surface which may result in reduced electrical performance.
Other concepts have been proposed to minimize the invasive nature of defibrillation leads. To date, these approaches have involved incorporation of some form of active fixation device in the lead. Drawbacks of these approaches include limitations on the type of fixation mechanisms that can be used and the necessity that each lead be of a custom design to incorporate its own fixation mechanism. This, of course, complicates the design of the leads. Thus, there remains a definite need to increase the efficiency of implant defibrillation devices while, at the same time, avoiding the problems associated with tissue damage or trauma, tissue ingrowth and the necessity to design complicated fixation mechanisms.
In addition, in order to place the patch electrode on or adjacent to the left ventricle or other desired area of the heart, it has been necessary, with some procedures, for the patient to undergo extensive surgery involving substantial recovery time. The most radical approach involves splitting the sternum or alternatively opening a space between the ribs in order to gain access to the surface of the heart. In a different, less traumatic, surgical approach, a small incision is made from beneath the sternum and the thoracic cavity is entered directly from beneath the rib cage. To date, this approach has not met with a great deal of success, however, because of the limitations of present techniques with respect to proper placement and fixation of the electrodes. Remote manipulation is difficult and a specific mode of attachment designed for remote operation has not been devised.